Data Communications Networking Devices - 4th Ed.pdf

128 ______________________________ FUNDAMENTAL WIDE AREA NETWORKING CONCEPTSemployed to generate the checksum is easy to implement using a higher levellanguage such as BASIC or Pascal. In comparison,a CRC-16 block-checkcharacter is normally generated using assembly language.As a result of the previously mentioned limitations associated with theXMODEM protocol,several extensions to that protocol were developed. Inaddition,many commercial software developers designed proprietary ®le transferprotocols that were also structured to overcome one or more of the limitationsassociated with the XMODEM protocol. Five of the more popular extensions ofthe XMODEM protocol are XMODEM/CRC,YMODEM,YMODEM-G,XMODEM-1K and ZMODEM. Some of those protocols also have what areknown as batch extensions which support the transfer of multiple batched) ®les.Concerning commercial proprietary software protocols,two of the more popularare the BLAST and CrossTalk protocols. To provide readers with an indication ofthe advantages and disadvantages of each protocol we will compare and contrastseveral of those protocols to the original XMODEM protocol.XMODEM/CRC protocolThe XMODEM/CRC is very similar to the XMODEM protocol,except that atwo-byte CRC-16 is used in place of the one-character arithmetic checksum usedwith the original protocol. To differentiate the use of a CRC-16 from the use of achecksum,the receiver speci®es the CRC-16 by transmitting the character C Hex43) instead of a NAK when requesting the ®rst packet.Figure 1.72 illustrates the block format of the XMODEM/CR protocol. Incomparing the format of the XMODEM/CRC protocol to the XMODEMprotocol,you will note the similarity between the two protocols,since only theerror detection mechanism has changed.Figure 1.72XMODEM/CRC block formatXMODEM/CRCThrough the use of a CRC,the probability of an undetected error is signi®cantlyreduced in comparison to the use of the XMODEM checksum. The CRC willdetect all single- and double-bit errors,all errors with an odd number of bits,allbursts of errors up to 16 bits in length,99.997% of 17-bit error bursts,and 99.998%of 18-bit and longer bursts.Although the XMODEM/CRC protocol signi®cantly reduces the probability ofan undetected error,it is a half-duplex protocol similar to XMODEM and uses thesame size data block. Thus,it removes only one of the three constraints associatedwith the XMODEM protocol.